Add tests for buffered http get

[janos]

No description provided.

[acud]

@janos this generally LGTM however I have the following concerns:

• i'm not sure that this is exhaustive with the original ioCopyBufferSize - 32kb is not that much, taking into account os caching and os native kernel reader buffer sizes. my assumption they will vary greatly in different systems under different hardware conditions. the variance in uploaded file size is also not big enough IMHO. to get a proper regression I'd aim for bigger numbers and have the test run for a longer time.
• flakyness alert - you don't think this will flake quite often? are the slope values travis adjusted? i'm usually not in favour of having hardcoded relative result values running on a CI - this asks for trouble. boolean assertions are better

[janos]

@justelad Thanks for the review.

1. The buffer that is in question is this one here https://github.com/golang/go/blob/master/src/io/io.go#L391 which is fixed and we are addressing with buffered read only this issue, regardless of other caching on lower levels.

Would you like to add a -longrunning variation of tests for bigger files? This sizes with slower get's are in order of magnitude that are acceptable for standard tests on Travis.

2. I have adjusted the slope values to be more tolerant for Travis in the latest commit. I could not think any boolean assertion that would satisfy the performance change detection. Do you have some suggestions beside linear regression?